Optical signals do not diffract as much as radio frequency (RE) signals. This makes them useful for a variety of ranging and radar-like imaging applications using relatively small apertures. One common light detection and ranging (lidar) method is a phase-shift method here an optical signal is modulated by a RF frequency, and the phase of the RF frequency of the return optical signal (the return optical signal is the signal that returns to the transceiver after bouncing off an object to be measured) is measured thus giving information about the distance to the object. This information is equivalent to a temporal delay measurement of the optical signal, as distance and time are related by the speed of light. The distance to the object is known to within some unambiguous range that in this case is proportional to the inverse of the RF frequency. The phase can be monitored over time (a phase change with respect to time is equivalent to an RF frequency shift) to determine the speed of the object with respect to the transceiver.
Avalanche photodiodes (APDs) are sometimes used as the optical detector since they have a large internal gain making them sensitive to the small levels of return light typically encountered, especially when the transceiver and object are far apart. The electrical signal from the APD can be mixed with an RF local oscillator in a mixer to translate the received signal frequency down to a level where signal processing can more easily be performed.
It would be advantageous in terms of sensitivity to use a single photon sensitive detector (SPD). However, such detectors have outputs that are not generally linear with respect to the input optical signal (for instance they may have binary digital outputs), thus a traditional mixer is not necessarily a preferred component for processing the SPD output. APD's can be operated in a SPD mode (the Geiger mode) where they are sometimes used in lidar to measure the time-of-flight of a pulse from a transmitter to the object and back again because of their very high sensitivity to small levels (single photons) of reflected light. The time-of-flight can be translated into distance to the object since the speed of light is constant. The optical pulse repetition rate in a time-of-flight scheme is typically quite low, making it difficult to perform velocity measurements or fast measurements suitable for imagine when using SPDs. The low pulse rate is due to a variety of factors, including the desire to have a long range over which the distance to the object can be measured unambiguously.
A method of measuring return optical signals that makes use of a time gated photon detector and a pulsed optical source with an optical pulse rate related to but unequal to the gate rate is described in provisional patent 13768652 “System and method for measuring the phase of a modulated optical signal.” The technique of using a related but unequal rate for the temporally-gated photon detector and for the optical pulse rate offers various advantages including a capability for high speed and high resolution measurements using practical components. The electronics required in this configuration can be simpler than other techniques since the digital output from the SPD can be processed without attempting to determine the exact time of the breakdown with high resolution (e.g. no high resolution time-to-digital converter is required). Since the time of the breakdown is localized by the temporal response of the time-gated detector and this temporal response can be narrower than typical detector jitter, this method can be used to determine the temporal locations of the single photon detection events with high resolution. One technique for generating very narrow time gates include temporally gating the SPD (U.S. patent application Ser. No. 13/768,652).
Another type of SPD uses a nonlinear nonlinear interaction with an optical pump to change the wavelength of a desired signal. This type of SPD is sometimes called an up-conversion SPD, and can be useful for changing the wavelength of the signal to one that is well matched to high quality SPD technology. It is possible to pulse the optical pump (“Up-conversion single-photon detector using multi-wavelength sampling techniques.” Optics express 1.9.6 (2011): 5470-5479), and this would produce a kind of time-gated SPD.
Lidar systems operate such that the distance to an object (or equivalently the time delay to and from the object) is measured to within some unambiguous distance. In typical time-of-flight lidar this distance is set by the pulse repetition rate, forcing a low repetition rate for objects that are far away. The lower repetition rate can be inconvenient for many reasons including longer measurement times and higher peak power levels. Some methods to extend the unambiguous range have been developed, including modulating the pulse sequence with a pseudo-random code (“Photon Counting Pseudorandom Noise Code Laser Altimeters,” Proc. SPIE Vol. 6771, X. Sun el. al., 2007). Adding, such modulation can add cost and complexity to the system.
While an improvement to the state of the art, this prior art can be expanded upon. What is needed is a high speed optical signal delay measurement system that is capable of operating over a wide range of received power levels, can obtain high resolution measurements with large unambiguous ranges in short measurement times, can measure multiple optical signals with a single or limited number of SPDs, and can work with a variety of SPD technologies. It is beneficial if the processing can be performed in real time, possibly employing an adaptive method to determine the measurement interval on which to process the data, including the option of stopping the data processing if the data quality is determined to be too inaccurate to expect a positive result thus conserving power and processing time. The raw data can also be stored and post-processed at a later time using multiple methods.